The present invention relates to encoding of video signals, and more particularly to a video quality improvement method and apparatus for correcting the brightness of a luminance channel due to brightness information lost during quantization of chrominance in the chrominance channels of an encoder.
For reasons of economy most television transmission, storage and processing means use a lower performance channel for the chrominance signal C than for the luminance signal Y of component video. The chrominance signal approximately represents the way that an element of a picture image is colored. The luminance component signal approximately represents the apparent brightness of that same picture element. This is in accordance with the well-known observation that the human eye is more tolerant of errors in color than of errors in apparent brightness. However also for reasons of economy the primary color signals GBR are "gamma corrected", which compresses their dynamic range to improve the signal-to-noise ratio for low brightness elements at the expense of a lessened signal-to-noise ratio for high brightness elements. This is in accordance with the well-known Weber-Fechtner relation that represents the dynamic response of the human eye as being approximately logarithmic.
Since the Y and C signals are derived from gamma-corrected primary color signals using a linear matrix, the C signal does substantially contribute to the apparent brightness of a picture element, particularly in the case of certain highly saturated colors. Additionally some television systems use a linear matrix that is not derived from the luminance coefficients for the phosphors of a CRT display, resulting in errors in the apparent brightness of even neutral colors when C signal errors are present. This brightness noise produces objectionable artifacts in the displayed picture, and has long been understood to be a consequence of the economies gained through Y-C transmission and gamma correction. Indeed, so long as the errors are noise, i.e., nondeterministic, they are uncorrectable.
FIGS. 1 and 2 illustrate the expected root-mean-square noise in apparent brightness due to unit amounts of noise in the Pb and Pr chrominance component signals, which are the two components of the C signal. The numbers "709" and "601" in the titles of these figures refer to the video standards from which the matrix coefficients are taken. The 709 standard is an orthogonal standard and the 601 standard is a non-orthogonal standard, as is well known in the art. To understand the significance of this incidental brightness noise, labeled LUM in the figures, the corresponding behavior when noise is added to the Y signal channel is examined.
When the same amount of error signal is added to the Y channel, approximately unit errors in apparent brightness result when using a Y signal amplitude of 0.5, as shown in FIGS. 3 and 4. Thus the incidental errors depicted in FIGS. 1 and 2 may result in substantial impairment of picture quality, particularly if the signal transmission quality of the C channel is impaired relative to that of the Y channel, as is usually the case. Further unless photometric measurements are made at the display, this type of impairment, although visible, may go unmeasured.
Some forms of video encoding, such as video compression, may involve the addition of deterministic errors, such as roundoff errors, to the C channel. These error signals are the result of quantizing the chrominance component signals, and may be available at the encoder as the C channel signal is being generated. The contributions of errors in the chrominance component signal channels to the perceived brightness are depicted in FIGS. 5-8. These figures depict the partical derivatives of brightness versus noise in the Pb and Pr channels for a Y signal amplitude of 0.5, which for neutral colors should result in a display brightness of about 0.26 of the brightness of reference white, i.e., Y=1.0.
What is desired is an encoder that anticipates the true brightness information that is lost in the chrominance channels due to encoder quantization and then applies an appropriate correction to the luminance channel before transmission.